How to adjust a car idle

London's flamboyant mayor, Boris Johnson, has recently declared war on those motorists who pollute the city streets by idling. Pehaps you're not exactly sure what can be done about preventing harmful fumes being emitted while you wait at traffic lights, or at the drive-through window. But if you've never thought about how to adjust your car idle, here's what you should consider.

Adjusting a car idle

Before we get round to focussing on how to adjust a car idle, it would be worth outlining what happens when vehicles are idling. When your engine is running while the car isn't actually going anywhere, as in waiting for that aforemetioned red lights to change, then your engine is still working away, running without any loads bar the engine accessories.

The reasons why this should be of any concern are mostly down to the potentially harmful effects of all those exhaust fumes being generated while the car remains stationary. Idling pollutants can have acute or chronic effects, ranging from irritation to the eyes and throat, an increased risk of adverse heart issues, right down to damage to sperm chromatin and DNA.

Few drivers ever consider there is anything wrong with idling at that drive-through window, or while waiting to pick someone up, rather than switching off. But here's an eye-popping statistic for you: an estimated 3.8 billion litres of fuel a year is consumed by idling vehicles in the USA.

Countering idling

So far, the main strategies aimed at countering the effects of idling have been based on encouraging drivers to be more economic about fuel consumption. Vehicles are also being increasingly designed with built-in power units that enable engine accessories to run more efficiently while idling.

The older your vehicle, the more its engine will either have a higher or lower idle speed than is required. This wastes fuel, or provokes stalling. The best way to resolve this it to take your vehicle into a garage to have it fine-tuned by a professional.

Quick guide to adjusting a car idle

Here is a quick overview on how to adjust a car idle:-1. Start the engine. Allow it to cycle through one period at a higher rev-per-minute rate. Once the engine is ticking over you'll be able to make the adjustment.

2. Find the throttle body (trace the air intake hose as it exits the air filter). Search on the body's sides until you find a screw, or rubber casing housing a screw. This is where you adjust the car idle.

3. Disconnect the idle air adjustment valve (which opens or closes depending on air intake) by locating the electrical connector behind the throttle body.